394 ADVENTURES IN RADIOISOTOPE RESEARCH 



by plasma P between 1 hour and 4 hours after the start of the experi- 

 ment. 



The fact that the inorganic P of the tissue is partly of extracellular 

 origin will lead to an overestimation of the activity of the cellular inor- 

 ganic P and, thus, to an underestimation of the renewal figures of the 

 organic P compounds. This source of error is mainly to be considered 

 in experiments of short duration carried out at low temperature. On 

 the other hand, even if the greatest precautions are observed, we risk 

 a decomposition of some of the creatine phosphate present in the tissue 

 prior to the separation of the inorganic P. Such a decomposition will 

 lead to a decrease in the specific activity of the inorganic P, the inor- 

 ganic P originating from creatine P being on the wdiole less active than 

 the "free" phosphate P. We shall, thus, underestimate the specific 

 activity of the inorganic P and, correspondingly, overestimate the rate 

 of renewal of the organic P compounds. This error will also be larger 

 in experiments of short duration carried out at low temperature. We 

 wish to mention a further possible experimental error. If the free phos- 

 phate is not precipitated quantitatively, we risk to find some strongly 

 active phosphate in the creatine phosphate fraction. A non-negligible 

 amount of phosphate may remain in solution in cases in which the 

 amount of P to be precipitated is very small. 



The following objection can be put forward regarding the calculation 

 of renewal rates from the ratio of the specific activities of the inorganic 

 P and the P split off from organic compounds. The P secured as inorganic 

 phosphate might even after the most careful handling of the tissue 

 have been largely present not as free phosphate in the tissue cells but 

 incorporated in very labile compounds which were decomposed in the 

 course of the extraction process. It is possible that this is the case, it 

 is even quite possible that a large part of the inorganic P extracted 

 as such from the muscle cells was originally present incorporated in very 

 labile compounds and was decomposed during the extraction process. 

 General experience indicates, however, that very labile phosphorus 

 compounds are renewed at a fast rate and we can, therefore, expect 

 the P of such labile phosphorus compounds to obtain within a short 

 time a similar specific activity as shown by the inorganic P present 

 in the cells. Should that not be the case, then the comparison of the 

 specific activity of the "inorganic" P with that of the P split off from 

 the organic compound in question, would obviously lead to an overesti- 

 mation of the rate of renewal. 



The specific activity of different P fractions is seen in Tables 11 — 18. 



Though all precautions were taken to prevent decomposition of 

 creatinephosphoric acid it is difficult to state whether the variations in 

 the values obtained for the rate of renewal of creatinephosphoric acid 

 molecules in some of the experiments are genuine or are due to a more 



